<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>after解决高度坍塌问题</title>
    <style>
        .box1{
            border:10px red solid;
        }
        .box2{
            width:100px;
            height:100px;
            background-color: #bfa;
            float: left;
        }
        .box1::after{
            content:"";
            display: block;
            clear:both;
        }
        .box3{
            width:200px;
            height:200px;
            background-color: khaki;
        }
    </style>
</head>
<body>
    <div class="box1">
        <div class="box2"></div>
        <div class="box3"></div>
    </div>
</body>
</html>